1. Field of the Invention
The invention relates to a tubular food casing based on regenerated cellulose having a coating on its inner surface, and to processes for its production and use. The casing is particularly suitable as an artificial sausage casing.
2. Description of Related Art
Fiber-reinforced food casings based on regenerated cellulose having a poly(vinylidene chloride) (PVDC) coating applied to the outside or inside are known. Coating the outside is technically simpler to accomplish. In contrast, the internally coated casings offer advantages in later processing. This is because, after filling with sausage emulsion, they can release water without impairment and thus shrink. The sausage thus constantly retains a full shape. It is also possible to use an externally coated casing, but this requires more complex equipment. This process has therefore not been able to establish itself.
One problem with the PVDC coating is that the softener (glycerol) is dissolved out of the cellulose hydrate casing by the coating dispersion and accumulates in the dispersion. On laying flat and also shirring of the casing, the coating is extremely mechanically stressed. The coating is thus frequently permanently damaged at edges and folds formed during this.
A dispersion suitable for the internal coating of cellulose hydrate casings, which dispersion comprises as essential constituent a copolymer having a content of vinylidene chloride (VDC) units of 86 to 92% by weight is disclosed in DE-B 25 12 994. Using this dispersion, a coating may be produced which already substantially possesses the required properties with respect to extensibility and strength. These properties of the coating change only slightly during storage. At a VDC content of 86 to 92% in the copolymer, the optimum film properties are only achieved at a very high molecular weight, however. Suitable copolymers are therefore only those whose 1% strength by weight solution in tetrahydrofuran (THF) has a relative viscosity xcex7rel of greater than 3.0.
During a mean storage time of about 8 to 10 days until shirring, the film attains its final structure and thus sufficient strength, so that the shirring folds are not damaged, even during any desired period of storage of the shirred sticks. The film then does not change any more during further storage. The lay-flat edges which are impressed in the still-warm film on leaving the machine are, in contrast, damaged to a certain extent during storage. They are inevitably impressed into the film too early, as a result of which the film is deformed. On further storage, the deformation increases up to intense damage at this point, which increases correspondingly during extension of the soaked casings.
The high molecular weight required for these copolymers can only be achieved by a slow polymerization with cooling. However, these conditions lead to monodisperse dispersions. Internal coatings thus produced become hazy on heating in water, as is customary on simmering of sausages. Relatively large pores first form in which glycerol is included. The glycerol is then dissolved out from the outside, through the fibrous casing. The remaining cavities appear as haze after drying. This hot-water haze gives the appearance of an unwanted fat deposit on the sausages.
A tubular casing of regenerated cellulose having a gas- and moisture-impermeable barrier layer on the inside or outside is described in DE-B 23 38 418 (U.S. Pat. Nos. 3,886,979 and 4,026,985). The barrier layer, at 0xc2x0 C., has an extensibility of at least 4%. The barrier layer can be produced from a vinylidene chloride copolymer, polyethylene or another polyolefin, polyester, polyamide, polyurethane or a combination thereof. There are no further details on the VDC copolymer.
DE-B 29 06 118 (U.S. Pat. No. 4,267,094) relates to a polymer blend from which hot-sealable layers having good resistance to blocking and good surface slip may be produced on cellulose hydrate fibrous casings. The blend consists of a component containing VDC copolymer and a polymeric anchoring component containing functional groups, which component comprises units of unsaturated carboxamido-N-methylol derivatives.
An artificial casing based on regenerated cellulose support material which has an inner coat of PVDC, polyolefin or nitrocellulose is disclosed in DE-B 13 02 384 (U.S. Pat. No.3,369,911). An adhesive intermediate layer is also mentioned. However, more detailed information on the PVDC is absent.
Fiber-reinforced and non-fiber-reinforced cellulose hydrate casings having a moisture-and air-impermeable coating are also described in DE-B 19 17 365 (U.S. Pat. No. 3,794,515). The coating is produced by application of a solution of a VDC homopolymer or VDC copolymer, a polyisocyanate or polyester having free isocyanate groups, a reaction product of a polyol with ethylene oxide, propylene oxide or butylene oxide in a low-boiling ether, ester or ketone, drying the layer thus produced and subsequent hardening by heating.
A cellulose hydrate casing having a chlorine-free polyacrylate coating is also disclosed in EP-A 0 457 178 (U.S. Pat. No. 5,316,809) which, due to the ready fusion of the dispersed particles, always forms a haze-free film (without glycerol inclusions). From a certain molecular weight, the shirring folds and edges are not damaged during storage. Increasing the coating weight, moreover, compensates for the poorer water vapor barrier, so that in the case of simmering sausage, the loss in weight is no higher than with a PVDC-coated fibrous casing. However, an adequate oxygen barrier cannot be achieved by this means. Liver sausages discolor on the surface at the latest after 2 to 3 weeks by oxidation; a storage life of at least 6 weeks is required, however. In any event, an advantage of the chlorine-free, internally coated fibrous casings is that they are biodegradable and compostable, and they can be burned without releasing dioxin, even at temperatures below 600xc2x0 C.
As the VDC content in the copolymer increases, the barrier properties (water vapor permeability, oxygen permeability and aroma permeability) of the coatings produced therewith improve; however, faults which are caused by the complicated bond between cellulose hydrate and PVDC also increase. The improved properties of the coating are therefore not reflected in the quality level of the internally coated sausage casings, but rather decrease it. Thus coating faults and the damage of shirring folds and edges at a high VDC content are increasingly more difficult to avoid.
In the case of the VDC copolymers according to DE-B 25 12 994 (U.S. Pat. No. 4,233,341) which have a high VDC content (86 to 92% by weight of VDC), damage to the shirring folds is prevented by a correspondingly high molecular weight (relative viscosity: greater than 3.0), because shirring is not carried out until completion of structure formation (crystallization). However, in the case of the lay-flat edges, this crystallization process cannot be waited for. The lay-flat edges are always impressed too early into the still-warm film, as a result of which the film is deformed. The deformation increases during storage and leads, on extension, to visible fault areas on the sausage.
A further crucial drawback is variable hot-water haze on scalding, which is caused by monodispersity. The high molecular weight required for the desired properties at this high VDC content is only achieved by slow polymerization with cooling, which inevitably leads to monodispersity.
However, haze and faults are also dependent on the base material. The higher its permeability, the more readily, and the greater the degree, to which glycerol is taken up and removed from the polymer film which forms (from the inner surface). In addition, the higher its permeability, the better the film melts, so that the damage to edges or shirring folds decreases.
An object of the invention was therefore to develop PVDC copolymers which may be used to produce an inner coating which remains haze-free and strong and supple enough so that shirring folds and lay-flat edges are not damaged during any desired storage period. A quality level is to be achieved, as is otherwise achievable only by an outer coating having two layers one on top of the other. For an internal coating, this complicated compromise in properties with a copolymer is therefore sought, since on the inside, clearly, only a single coating is possible.
It is also an object to develop a copolymer whose film structure is independent of the particle size distribution. In particular, it should melt better than the types having a high VDC content. The coating produced therewith is, in addition, immediately after leaving the coating machine, to have sufficient strength and toughness so that the lay-flat edges are not damaged during storage. Hitherto, this was only possible using a coating based on (C1-C20)alkyl-(meth)acrylate copolymers, as described in EP-A 457 178. Edges and shirring folds of the casings thus coated are undamaged after any desired storage periods and the coating remains haze-free on scalding. A disadvantage of a coating using (meth)acrylate copolymers, however, is its relatively strong tendency to blocking, despite the high molecular weight (relative viscosity: greater than 3.0). It is also an object to produce an internally coated fibrous casing that has a high water vapor barrier (weight loss of 1 kg of meat sausage in a caliber-60 casing: less than 1% per week) and a high oxygen barrier (no discoloration of calf s liver sausage after a storage time of 6 weeks). In addition, it is desired that the shirring folds and lay-flat edges remain undamaged with any desired storage period and that the inner coating does not show hot-water haze on scalding.
Surprisingly, it has been found that the objects can be achieved by a tubular food casing based on regenerated cellulose having an internally coated surface, wherein the coating comprises at least one copolymer having 50 to 84% by weight of vinylidene chloride units, 2 to 20% by weight of acrylonitrile units, 1 to 10% by weight of acrylic acid units and 1 to 47% by weight of (C1-C18)alkyl (meth)acrylate units.
In accordance with the invention there is also provided a process for producing the food casing, which comprises applying an aqueous dispersion of the copolymer to the inner surface of a tubular casing based on regenerated cellulose, which casing is optionally fiber-reinforced, and subsequently drying the casing.
In accordance with the invention, there is also provided a copolymer having 50 to 84%, by weight of vinylidene chloride units, 2 to 20% by weight of acrylonitrile units, 1 to 10% by weight of acrylic acid units, and 1 to 47% by weight of (C1-C18)alkyl (meth)acrylate units, based on the total weight of the copolymer.
Further objects, features, and advantages of the invention will become apparent from the detailed description that follows.